This application claims the priority of German Application No. 101 61 850.6-13, filed Nov. 15, 2001, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for operating an internal combustion engine of a motor vehicle having an engine control unit and an exhaust-gas catalytic converter.
To reduce the fuel consumption and to lower the emission of pollutants in motor vehicles, it is known to automatically switch off the internal combustion engine when the vehicle is temporarily stationary. When the driver of the motor vehicle wants to proceed, the internal combustion engine will be started up again in response to a suitable signal (what is known as automated start stop operation). The automatic switching off and restarting are usually initiated by an engine control unit which also determines that the vehicle is stationary and that the driver wishes to proceed. In this way, the operating time of the internal combustion engine is reduced, and therefore the fuel consumption and the emission of pollutants from the motor vehicle are also reduced.
It is an object of the present invention to provide a method for operating an internal combustion engine of a motor vehicle in which the emission of pollutants from the motor vehicle is still further reduced in combination with the lowest possible fuel consumption.
The method according to the invention is distinguished by using an engine control unit to switch off the internal combustion engine in an idling mode and/or overrun mode as a function of the temperature of an exhaust-gas catalytic converter. The decision as to whether to switch off the internal combustion engine during idling and during an overrun mode is accordingly made as a function of the current temperature of the exhaust gas and the current temperature of the exhaust-gas catalytic converter. This avoids switching off the internal combustion engine at a time which is unfavourable for operation of the exhaust-gas catalytic converter. For example, after a short journey following a cold start of the motor vehicle, the exhaust-gas catalytic converter is generally not yet at operating temperature, whereas the internal combustion engine is already providing significantly heated exhaust gas. If, at such a time, the vehicle comes to a standstill and the internal combustion engine is switched off, the further heating of the exhaust gas catalytic converter is delayed, so that it continues to be unable to function fully and consequently cannot purify or cannot sufficiently purify the exhaust gas after the internal combustion engine has been started up again and the trip has once again commenced. Therefore, in the operating method described above, it is advantageous for the overall emission of the pollutants from the motor vehicle if the internal combustion engine is switched off in idling mode while the vehicle is stationary only as a function of the temperature of the exhaust-gas catalytic converter and of the exhaust-gas temperature. Similarly, the emission of pollutants from the motor vehicle is likewise reduced if, when the internal combustion engine is in overrun mode, it is switched off as a function of the temperature of the exhaust gas catalytic converter and of the exhaust-gas temperature.
In one configuration of the invention, the internal combustion engine is switched off in idling and/or overrun mode if the temperature T of the exhaust-gas catalytic converter lies within a range with a predetermined lower limit temperature T1 and a predetermined upper limit temperature T2. It is preferable for the lower limit temperature T1 to be linked to what is known as the light off temperature of the catalytic converter, i.e. the temperature above which a certain minimum conversion of pollutants takes place. The upper limit temperature T2 is preferably determined by the thermal stability limit of the catalytic converter. This on the one hand prevents the catalytic converter from being operated at below its light-off temperature for longer than necessary or more often than necessary. Therefore, this measure improves the overall emission of pollutants from the vehicle. On the other hand, the catalytic converter is prevented from being operated for longer than necessary or more often than necessary at above its thermal stability limit. Therefore, possible damage to the catalytic converter is avoided, and consequently its purification function is retained for a longer time. If there are a plurality of exhaust-gas catalytic converters in the exhaust-gas purification system, it is preferable for limit temperatures to be taken into account for each catalytic converter and for the internal combustion engine only to be switched off if the temperature of each catalytic converter lies within the associated temperature range, i.e. between T1 and T2.
In a further configuration of the invention, the internal combustion engine is switched off in idling and/or overrun mode if the temperature of the exhaust-gas catalytic converter lies below a predetermined lower limit temperature T1 and, at the same time, the temperature of the exhaust gas flowing into the exhaust-gas catalytic converter is less than or equal to the temperature of the exhaust-gas catalytic converter. This measure prevents the exhaust-gas catalytic converter being cooled further by the exhaust gas, so that it loses efficiency as a result. Therefore, it is preferable for the limit temperature T1 to be selected to be approximately equal to the light off temperature of the corresponding catalytic converter.
In a further configuration of the invention, the internal combustion engine is switched off in idling and/or overrun mode if the temperature of the exhaust-gas catalytic converter lies above a predetermined upper limit temperature T2. It is preferable for this second limit temperature T2 to lie in the region of the thermal stability limit of the catalytic converter. Since in idling or overrun mode exhaust gas which is at a relatively low temperature is supplied by the internal combustion engine, in this case the exhaust gas cools the greatly heated catalytic converter and therefore prevents it from becoming damaged. The situation which has been outlined may, in practical driving conditions, occur, for example, if, following full-load operation with considerable heating of the catalytic converter, the vehicle temporarily comes to a standstill. Since, according to the invention, in this situation the internal combustion engine is not switched off, the catalytic converter is cooled again relatively quickly by the exhaust gas in idling mode, which is only heated to a relatively minor extent. Therefore, the inventive measure means that the catalytic converter is exposed to a lower thermal load, and therefore the natural aging process slows.
In a further configuration of the invention, the internal combustion engine is not switched off in idling and/or overrun mode if the temperature of the exhaust-gas catalytic converter lies below a predetermined lower limit temperature T1 and, at the same time, the temperature of the exhaust gas flowing into the exhaust gas catalytic converter is higher than the temperature of the exhaust-gas catalytic converter. It is preferable for the lower limit temperature T1 to be selected to be approximately equal to the light-off temperature of the corresponding catalytic converter, and consequently the result of this measure is that the catalytic converter reaches its active temperature range as a result of the heating effected by the exhaust gas and can then perform its exhaust-gas purification function.
In a further configuration of the method according to the invention, the internal combustion engine is switched off in idling and/or overrun mode only after a predetermined waiting time after the engine control unit has determined that the engine should, in principle, be switched off according to the conditions cited above. The waiting may, for example, be 5 seconds, which prevents the internal combustion engine from being switched off and started up again repeatedly as a result of driving and standstill conditions following one another in very close succession, which would lead to increased wear to the starter device.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.